Magnetic model for A2CuP2O7 (A= Na, Li): One-dimensional versus two-dimensional behavior

We report magnetization measurements, full-potential band-structure calculations, and microscopic modeling for the spin-1/2 Heisenberg magnets ${A}_{2}$Cu${P}_{2}$${O}_{7}$ ($A=\text{Na}$, Li) involving complex Cu-O-O-Cu superexchange pathways. Based on a quantitative evaluation of the leading exchange integrals and the subsequent quantum Monte Carlo simulations, we propose a quasi-one-dimensional magnetic model for both compounds, in contrast to earlier studies that conjectured on a two-dimensional scenario. The one-dimensional nature of ${A}_{2}$Cu${P}_{2}$${O}_{7}$ is unambiguously verified by magnetization isotherms measured in fields up to 50 T. The saturation fields of about 40 T for both Li and Na compounds are in excellent agreement with the intrachain exchange ${J}_{1}\ensuremath{\simeq}27$ K extracted from the magnetic susceptibility data. The proposed magnetic structure entails spin chains with the dominating antiferromagnetic nearest-neighbor interaction ${J}_{1}$ and two inequivalent, nonfrustrated antiferromagnetic interchain couplings of about 0.01${J}_{1}$ each. A possible long-range magnetic ordering is discussed in comparison with the available experimental information.